Semiconductor laser device

ABSTRACT

A semiconductor laser device comprising a substrate of a first conductivity type having a mesa; a first semiconductor layer of a second conductivity type which is formed on the upper surface of the substrate other than the mesa to form a flat plane including the top face of the mesa; a laser oscillation region which is formed on the flat plane and includes an active area for laser oscillation; and a multi-layer structure burying the laser oscillation region, the multi-layer structure comprising a high resistance layer formed on the first semiconductor layer and burying both sides of the laser oscillation region, and a second semiconductor layer of the first conductivity type formed on the high resistance layer.

BACKGROUND OF THE INVENTION

1. Field of the invention

This invention relates to a semiconductor laser device which has anextremely low threshold current level and can be readily produced bymolecular beam epitaxy or metal organic-chemical vapor deposition.

2. Description of the prior art:

In recent years, single thin crystal film growth techniques such asmolecular beam epitaxy (MBE) and metal organic-chemical vapor deposition(MOCVD) have been rapidly advanced. By these growth techniques, it ispossible to obtain epitaxial growth layers of extreme thinness, on theorder of 10 Å. Due to the progress in these crystal growth techniques,it is possible to make laser devices based on device structures havingvery thin layers, which could not be easily manufactured by conventionalliquid phase epitaxy. A typical example of these laser devices is thequantum well (QW) laser which has an extremely low threshold currentlevel and in which the active layer has a thickness of 100 Å or less,resulting in the formation of quantum levels therein. Particularly, asingle quantum well laser device of a GRIN-SCH (Graded Index-SeparateConfinement Heterostructure) type has a very low threshold currentlevel, for example, 350 A/cm² (cavity length: 250 μm) and 200 A/cm²(cavity length: 500 μm). This has been reported by W. T. Tsang, AppliedPhysics Letters, vol. 40, 1981, p. 217.

Further, in order to operate a semiconductor laser device at a lowcurrent level, the injected current must be confined in the laseroscillation region. Known examples of semiconductor laser devicesshowing such a current confining function are a VSIS (V-channeledSubstrate Inner Stripe) laser device and a BH (Buried Heterostructure)laser device.

FIG. 4 shows a cross sectional view of a conventional VSIS laser device.Referring to FIG. 4, the structure of the VSIS laser device and themethod of producing the device will be described.

On the surface of a semiconductor substrate 1, a current blocking layer6 is formed. A V-channel which reaches to the substrate 1 is formed inthe current blocking layer 6 by an etching technique to form a narrowcurrent path. Then, on the current blocking layer 6 including theV-channel, a cladding layer 2, an active layer 3, a cladding layer 4 anda cap layer 5 are successively formed by a liquid phase epitaxy (LPE),which shows very excellent characteristics of covering a surface havinga step(s). As a result of forming the V-channel, the current path ismade narrow so that the unstable oscillation mode is eliminated and thedevice oscillates in the fundamental traverse mode. The VSIS laserdevice in which the flat active layer 3 is sandwiched by the claddinglayers 2 and 4 to form a double heterojunction has a threshold currentof 40 mA or less when the width w of the V-channel is 4 μm.

Various kinds of index-guided semiconductor laser devices of this typehave been widely developed using an LPE technique. Such a laser deviceis produced by forming crystalline layers on a substrate having achannel or mesa. Therefore, it is usually difficult to produce such alaser device by MBE or MOCVD. A laser device of this kind has a defectthat the active region having a quantum well structure cannot be formed,thereby inhibiting the operation of the laser device at a low currentlevel.

FIG. 5 shows a cross sectional view of a BH laser device. Referring toFIG. 5, the structure of the BH laser device and the method of producingthe device will be described.

On the semiconductor substrate 1 of a first conductivity type, a lowercladding layer 2 of the first conductivity type, an active layer 3 and aupper cladding layer 4 of a second conductivity type are successivelyformed by MBE or MOCVD. Then, the wafer is etched to form a mesa-shapeddouble heterojunction laser oscillation region including the claddinglayers 2 and 4 and active layer 3. A first burying layer 8 of a secondconductivity type and a second burying layer of a first conductivitytype are successively formed on the surface of the substrate 1 to burythe oscillation region. Then, a cap layer 5 is formed. The refractiveindexes of the burying layers 8 and 9 are selected to be smaller thanthe effective refractive index of the laser oscillation region so thatthe laser light can be sufficiently confined in the mesa-shapedoscillation region.

As the burying layers 8 and 9 are reversely biased during the operationof the laser device, the injected current cannot pass the burying layers8 and 9 and are confined in the laser oscillation region, resulting inthe formation of a narrow current path. In this index-guided BH laserdevice, the current and light are confined within the oscillation regionby the burying layers 8 and 9, thereby achieving a laser device with alow operating current. A semiconductor laser device of this type havinga stripe width (a width of a mesa region) w of 2 μm or less can achievea low threshold current level of 10 mA or less.

The threshold current level of the BH laser device may be loweredfurther by replacing the active region with one having a quantum wellstructure and decreasing the width w. However, unless, the height of theinterface (p-n junction) of the first and second burying layers 8 and 9coincides with that of the interface between the active layer 3 and thesecond cladding layer 4, as shown in FIG. 4, it is very difficult tolower the threshold current level to about 1 mA because of the followingreason. When the height of the interface between the first and secondburying layers 8 and 9 fails to coincide with that of the interfacebetween the active layer 3 and the second cladding layer 4, paths ofineffective current which do not contribute to the laser oscillation areformed as shown by the arrows in FIGS. 6A and 6B. This ineffectivecurrent, which amounts generally to about 1 to 5 mA, is larger than thecurrent necessary for laser oscillation, causing the main factor ofinhibiting the production of a semiconductor laser of a low thresholdcurrent level.

Even if the height of the interface between the first and second buryinglayers 8 and 9 coincides with that of the interface between the activelayer 3 and the second layer 4, it is impossible to suppress the amountof the ineffective current flowing from the active layer 3 to theburying layer 8 or 9 to an extremely low level of about 1 mA or less.

As seen from the above, in conventional VSIS semiconductor laserdevices, it is very difficult to form an active area of a quantum wellstructure by MBE or MOCVD. In conventional BH semiconductor laserdevices, it is possible to form an active area of a quantum wellstructure by MBE or MOCVD, but it is very difficult to coincide theheight of the interface of the active area with that of the p-n junctionformed by the burying layers. Even if the heights are coincided witheach other, it is very difficult to prevent the leakage of the injectedcurrent from the active area to one of the burying layers. In any case,it is difficult to produce a semiconductor laser device of a lowthreshold current level by a conventional technique.

SUMMARY OF THE INVENTION

The semiconductor laser device of this invention, which overcomes theabove-discussed and numerous other disadvantages and deficiencies of theprior art, comprises a substrate of a first conductivity type having amesa, a first semiconductor layer of a second conductivity type which isformed on the upper surface of said substrate other than said mesa toform a flat plane including the top face of said mesa, a laseroscillation region which is formed on said flat plane and includes anactive area for laser oscillation and a multi-layer structure buryingsaid laser oscillation region, said multi-layer structure comprising ahigh resistance layer formed on said first semiconductor layer andburying both sides of said laser oscillation region, and a secondsemiconductor layer of the first conductivity type formed on said highresistance layer.

In a preferred embodiment, the active area comprises quantum welllayers.

In a preferred embodiment, the width of said active area issubstantially the same as the width of said mesa.

In a preferred embodiment, the laser oscillation area is formed as amesa shape.

In a preferred embodiment, the laser oscillation area is formed as areversed mesa shape.

In a preferred embodiment, the laser oscillation region is formed as amesa shape in which the width of the middle portion is smaller than thewidths of the top and bottom portion.

Thus, the invention described herein makes possible the objectives of(1) providing a semiconductor laser device which can prevent theineffective current from flowing through the device, resulting in anextremely low operation current level of the device; (2) providing asemiconductor laser device having an active area of quantum wellstructure which can be produced by MBE or MOCVD; and (3) providing asemiconductor laser device which can be readily produced in high yield.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention may be better understood and its numerous objects andadvantages will become apparent to those skilled in the art by referenceto the accompanying drawings as follows:

FIGS. 1A to 1D are cross sectional views showing diagrammatically oneexample of the present device at each of the production steps.

FIG. 2 is a diagrammatic cross sectional view of the example.

FIG. 3 is a cross sectional view of a GRINSCH semiconductor laserdevice.

FIG. 4 is a cross section view of a conventional VSIS semiconductorlaser device.

FIG. 5 is a cross sectional view of a conventional BH semiconductorlaser device.

FIGS. 6A and 6B are diagrams illustrating the current flows of thedevice shown in FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to the invention, the semiconductor laser device comprises thesubstrate of the first conductivity type having the mesa and the firstsemiconductor layer of the second conductivity type burying the steps ofthe substrate formed by the mesa, and the top face of the mesa and thesurface of the first semiconductor layer form a flat plane, therebyenabling the formation of the laser oscillation region on the flat planeby MBE or MOCVD.

The laser oscillation region is buried by the high resistance layer, andthe second semiconductor layer of the first conductivity type is formedon the high resistance layer. The first semiconductor layer, the highresistance layer and the second semiconductor layer are reversely biasedduring the operation of the laser device, and, therefore, the injectedcurrent is confined into the laser oscillation region. Further, thelaser oscillation region is buried by the high resistance layer, therebypreventing the ineffective current from flowing into the buryingstructure.

EXAMPLE

FIGS. 1A to 1D show the steps of producing one example of thesemiconductor laser device of the invention, and FIG. 2 shows a crosssectional view of the device. Referring to FIGS. 1A to 1D and 2, theproduction process and structure of the example will be described.

The surface of a p-GaAs substrate 11 is etched to form a stripe mesa 20having a width of 1 μm and a height of 2 μm. On the substrate 11, ann-GaAs current blocking layer 12 (Te=2×10¹⁸ cm⁻³) is grown so that themesa 20 is buried (FIG. 1A).

The upper portion of the current blocking layer 12 is etched away toexpose the top of the mesa 20, thereby forming a flat plane which iscomposed of the top portion 19 of the mesa 20 and the surface of thecurrent blocking layer 12, as shown in FIG. 1B.

As shown in FIG. 1C, on the flat plane, a p-Al₀.7 Ga₀.3 As claddinglayer 13 (Be=1×10¹⁸ cm⁻³ ; the thickness thereof being 1 μm), an undopedAl_(x) Ga_(1-x) As GRIN layer (optical guiding layer) 14 (the thicknessthereof being 0.2 μm), an undoped GaAs quantum well layer (active area)15 (the thickness thereof being 70 Å), an undoped Al_(x) Ga_(1-x) AsGRIN layer (optical guiding layer) 16 (the thickness thereof being 0.2μm), an n-Al₀.7 Ga₀.3 As cladding layer 17 (Si=1×10¹⁸ cm⁻³ ; thethickness thereof being 1 μm), and an n-Al₀.05 Ga₀.95 As cap layer 18(Si=1×10¹⁸ cm⁻⁻ ; the thickness thereof being 0.05 μm) are successivelygrown by MBE. In this example, the AlAs mole fraction x of the GRINlayers 14 and 16 is parabolically changed in the range of 0.2 to 0.7.Namely, the AlAs mole fraction of the GRIN layers 14 and 16 increaseparabolically with distance from the quantum well layer 15.

Then, the wafer is subjected to an etching treatment to form a stripemesa-shaped laser oscillation region including layers 13 to 18 (FIG.1D). By suitably selecting the etchant and employing one of conventionaletching techniques, the laser oscillation region can be shaped so thatthe cross section of the portion lower than the active area 15 has ashape of a normal mesa and that of the portion higher than the activearea 15 has a shape of a reversed mesa, as shown in FIG. 1D. The base ofthe mesa includes the top portion 19 and the portions 12a and 12b of thecurrent blocking layer 12 which are positioned on both sides of the topportion 19. In other words, the cladding layer 13 in the mesa-shapedlaser oscillation region is formed on the top portion 19 sandwiched bythe portions 12a and 12b. In the example, the width of the quantum welllayer 15 can be as narrow as about 1 μm because the lower portion of themesa-shaped laser oscillation region is formed as a normal stripe mesa.

Thereafter, an undoped Al₀.8 Ga₀.2 As burying layer 21 (0.1 Ω. cm ormore), a p-Al₀.8 Ga₀.2 As burying layer 22 (Mg=1×10¹⁸ cm⁻³), and an-GaAs cap layer 23 (Te=1×10¹⁸ cm⁻³) are successively grown by LPE so asto bury the mesa-shaped laser oscillation region. Both sides of thequantum well layer (active area) 15 are fully buried by the highresistance layer 21. Then, Au-Zn and Au-Ge/Ni are deposited on the backface of the n-GaAs substrate 11 and the upper face of the cap layer 23,respectively, and an alloy treatment is conducted to form ohmic p-sidedand n-sided electrodes 31 and 32, resulting in a semiconductor laserdevice.

The burying layer 21 which has a high AlAs mole fraction and acts as ahigh resistance layer can be readily formed by LPE. On the p-substrate11, the n-current blocking layer 12, the high resistance layer 21 andthe p-burying layer 22 are formed in this sequence. When the laserdevice is forwardly biased to be operated, this pin layer structure isreversely biased. Therefore, the current injected into the laser devicecannot flow through the pin layer structure, that is, the current flowsonly through the laser oscillation region, resulting in a narrow currentpath. Further, as the laser oscillation region is buried by the highresistance layer 21, the current does not leak out from the laseroscillation region (i.e., no ineffective current flows).

The semiconductor laser device thus produced and having a cavity lengthof 250 μm oscillates a laser beam of 840-nm wavelength at a thresholdcurrent of 1.5 mA.

FIG. 3 shows a cross sectional view of a GRIN-SCH semiconductor laserdevice. The device has a structure the same as that of theabove-mentioned example except that the current blocking layer is notformed and the laser oscillation region is not shaped into a mesa. Thethreshold current density of this GRIN-SCH laser device having a cavitylength of 250 μm is 360 A/cm². The ineffective current of the examplehaving the cavity width of 0.1 μm can be calculated as follows:

    1.5 mA-(360 A/cm.sup.2 ×1 μm ×250 μm)=0.6 mA

As seen from the above, the threshold current of the example issuppressed to a very low level, i.e., less than 1 mA.

In the above-described example, the laser oscillation is formed on thep-GaAs substrate. In stead of the p-GaAs substrate, a n-GaAs substratecan be used. In this case, the conductivity type of each layer should bechange adequately. The shape of the laser oscillation region is notlimited to that shown in FIG. 1D, but can be suitably modified. Further,the material to be used in the invention is not limited to AlGaAs, butInAlGaP, InGaAsP or the like can be used in the invention. The growth oflayers on the substrate can be conducted by a combination adequatelyselected from LPE, MBE, MOCVD and VPE (Vapor Phase Epitaxy).

It is understood that various other modifications will be apparent toand can be readily made by those skilled in the art without departingfrom the scope and spirit of this invention. Accordingly, it is notintended that the scope of the claims appended hereto be limited to thedescription as set forth herein, but rather that the claims be construedas encompassing all the features of patentable novelty that reside inthe present invention, including all features that would be treated asequivalents thereof by those skilled in the art to which this inventionpertains.

What is claimed is:
 1. In a semiconductor laser device comprising:asubstrate of a first conductivity type having a mesa; a firstsemiconductor layer of a second conductivity type which is formed on theupper surface of said substrate other than said mesa to form a flatplane including the top face of said mesa; a laser oscillation structurecomprising a first cladding layer, a first GRIN layer, a second GRINlayer, and a second cladding layer, which is formed on said flat planeand includes an active area for laser oscillation between said first andsecond GRIN layers and above said mesa; and a multi-layer structureburying said laser oscillation structure, said multi-layer structurecomprising a high resistance layer formed on said first semiconductorlayer and burying both sides of said laser oscillation structure, and asecond semiconductor layer of the first conductivity type formed on saidhigh resistance layer.
 2. A semiconductor laser device according toclaim 1, wherein said active area comprises quantum well layers.
 3. Asemiconductor laser device according to claim 1, wherein the width ofsaid active area is substantially the same as the width of said mesa. 4.A semiconductor laser device according to claim 1, wherein said laseroscillation structure is formed as a mesa shape.
 5. A semiconductorlaser device according to claim 1, wherein said laser oscillationstructure is formed as a reversed mesa shape.
 6. A semiconductor laserdevice according to claim 1, wherein said laser oscillation structure isformed as a mesa shape in which the width of the middle portion issmaller than the widths of the top and bottom portions.